A multi-fuel engine is an engine designed to combust multiple types of fuel with air in its operation. Multi-fuel engines may be desirable where cheaper and cleaner fuel sources, such as natural gas, are available as a primary fuel source but a secondary fuel (e.g., diesel) is desired for performance reasons or as a backup in the event of depletion of the primary fuel source. A dual fuel engine, for example, may be designed to run on either or both of a primary fuel source and a secondary fuel source at a range of relative ratios depending on performance requirements and availability of the fuel sources. In one implementation of a dual fuel engine, the engine may primarily run on natural gas with a pilot amount of diesel fuel being used as an ignition source.
Multi-fuel engines, such as dual fuel engines, require precise knowledge of the properties each of the fuel types in order to regulate various engine performance parameters such as the fuel flow rate and the engine air/fuel ratio, or lambda (λ). However, some gaseous fuel sources, such as natural gas, may have a composition that is unknown and/or changing with time. For example, natural gas may include various species such as methane (CH4), ethane (C2H6), propane (C3H8), and gases (e.g., nitrogen (N2) and carbon dioxide (CO2)) in various relative percentages that may continuously change with time. With the composition of natural gas being unknown, the physical properties (e.g., specific gravity, lower heating value (LHV), etc.) of the fuel will be unknown as well. For some applications, it may be cost-prohibitive to employ separate analytical techniques to continuously monitor the composition and properties of the natural gas fuel as it is being supplied to the engine. When running on one or more fuel sources with unknown properties, the performance of a multi-fuel engine may suffer. For example, it may be difficult or impossible to carefully regulate the fuel flow rate and λ if the specific gravity of one or more of the fuel sources is unknown.
U.S. Patent Application Publication Number 2013/0233056 teaches an analytical device that is capable of measuring the specific gravity of a gas fuel, such as natural gas, in a pipeline. The device disclosed therein is configured to measure the specific gravity of the gas fuel on the basis of the refractive index of the gas fuel and the speed of sound of the gas fuel. While effective, further improvements that enable property measurements of unknown fuel compositions are still wanting.
Clearly, there is a need for improved methods and systems capable of monitoring fuel properties in engines operating with a fuel source having an unknown composition.